1. Field of the Invention
The present invention relates to polyimide optical materials and an optical waveguide element where the polyimide optical materials ate employed. In particular, the present invention relates to polyimide which is useful as an optical material for the optical waveguide of an optoelectronic integrated circuit (OEIC) or of an optoelectronic mixed printed wiring board and to an optical waveguide element where such a polyimide is employed.
2. Description of the Related Art
Organic polymer materials are advantageous over inorganic materials, in that they are relatively light, excellent in impact resistance and workability, and easy to handle. Because of such advantages, organic polymer materials have been used extensively in optical components such as optical fiber, lenses, and substrate for optical disks.
There is a problem, however, if the polymer materials are employed as a medium for enabling a near-infrared ray for optical communication to transmit therethrough, for example as an optical waveguide of an OEIC or of an optoelectronic mixed printed wiring board, the problem being a relatively large light transmission loss as compared with inorganic materials. The causes for this can be generally classified into scattering and absorption. As the wavelength of light to be employed for the optical communication is shifted to the longer wavelength side (from 0.85 μm to 1.0 μm–1.7 μm), the infrared vibration which is intrinsic to the molecular structure of polymer materials is increasingly lost by harmonic absorption and this loss of the infrared vibration becomes predominant. As a result, it is now feared that the application of the polymer materials for use in the optical communication may become difficult.
In particular, polymethylmethacrylate (PMMA) and polystyrene (PS) which have been widely employed to date as an optical material for visible light, are formed of molecular structures having at least two carbon-hydrogen bond (C—H bond) in their molecules. Therefore, the near-infrared absorption spectra of these polymers include a plurality of absorption peaks which are large in width and intensity. To shift the harmonic absorption originating from this C—H bond toward the longer wavelength side, to minimize the intensity of the harmonic absorption, the employment of deuterium (D) or fluorine (F) to substitute for the intramolecular hydrogen has been proved to be effective. With respect to the materials where the hydrogen atoms in PMMA or PS are replaced by deuterium or fluorine, there have been already conducted fundamental studies. However, since these polymeric optical materials are not provided with sufficient soldering-heat resistance (260° C.) which is requisite in the fabrication of OEIC on the surface of a silicon substrate for instance, various measures will be necessitated in the fabrication process if the these polymeric optical materials are to be actually employed in the fabrication of OEIC, etc.
On the other hand, polyimide resin is known as ordinarily having a thermal decomposition temperature of as high as 400° C. or more, i.e. as one of the highest heat resistant resins among organic polymers, thus is now being studied to employ polyimide resin as an optical material.
For example, a coating material made of a fluorine-containing polyimide resin having hexafluoroisopropylidene group is now being studied for using it as a heat resistant material having an improved transparency. Further, there is proposed an optical waveguide formed of a fluorine-containing polyimide resin having hexafluoroisopropylidene group in the main chain thereof for the purpose of minimizing the light transmission loss.
However, the polyimide materials that have been conventionally studied, such as partially fluorinated polyimide, are accompanied with the following problems due to the fact that they contain a C—H bond of an aromatic ring in the molecular chain thereof. Namely, the near-infrared absorption spectra of these polyimide materials include a peak originating from the harmonics of the stretching vibration of C—H bond or from a combination vibration formed between the harmonics of the stretching vibration of C—H bond and the bending vibration. As a result, it is impossible to achieve the minimization of light transmission loss throughout the entire wavelength zone for optical transmission (1.0 μm to 1.7 μm).
Under the circumstances, it is now studied to employ, as a coating material, a fully fluorinated polyimide resin where all of hydrogen atoms in C—H bonds are substituted by fluorine atoms, and there is proposed an optical waveguide which is formed of this full fluorinated polyimide resin.
Although it is possible, with the employment of this full fluorinated polyimide resin, to achieve the minimization of light transmission loss throughout the entire wavelength zone for optical transmission (1.0 μm to 1.7 μm), the hydrogen radicals thereof are all substituted by fluorine atoms, resulting in an excess introduction of fluorine atoms. Accordingly, due to this excess fluorine group, the solvent resistance, as well as the heat resistance, which are characteristic of the conventional polyimide material, are caused to greatly deteriorate in this fully fluorinated polyimide resin, and at the same time, the workability thereof for forming optical elements as well as the solder reflow property thereof are caused to greatly deteriorate, thus obstructing the utility of this fully fluorinated polyimide resin as a material for the manufacture of the optical waveguide of an optical element.
Although other fluorinated polyimides have been proposed, no one has succeeded in providing an organic polymer material which is capable of meeting not only a high light transmitting property throughout the entire wavelength zone for optical communication but also other properties such as the solvent resistance and the heat resistance, all of which are required in the materials for manufacturing the optical waveguide of an optical element. Therefore, there is an earnest desire to develop an organic polymer optical material having all of the aforementioned properties.
Therefore, one of the objects of the present invention is to provide a polyimide optical material which is provided with sufficient heat resistance and solvent resistance demanded for the manufacture of an optoelectronic integrated circuit and which can be employed as an organic polymer optical material exhibiting an extremely low light transmission loss in the near infrared zone, in particular, throughout the entire wavelength zone for optical transmission (1.0 μm to 1.7 μm).
Another object of the present invention is to provide a polyimide precursor solution that is useful for synthesizing the aforementioned polyimide optical material.
A further object of the present invention is to provide an optical waveguide element which can be easily manufactured and is excellent in heat resistance and minimal in light transmission loss.